O'Flaherty returned from Tommy John surgery on July 3, but has been used more sporadically. He has not allowed a run over his last 10 1/3 innings covering his last 11 appearances in shrinking his ERA to 1.35.
Nieuwenhuis has neither started nor managed a hit since Aug. 8.
Pennington has earned the playing time. He has at least one hit in each of those starts and is 10 for 25 during that stretch with eight runs scored and six walks. His .369 on-base percentage is at its highest point since May 5.
Morrow tore a tendon sheath in his index finger in early May.
Donaldson has just three hits in 21 at-bats since slugging two home runs in Kansas City on Aug. 12.
Yet another setback in his recovery from a hamstring injury could prevent
Dirks reinjured his hamstring Sunday night while running the bases for Triple-A Toledo. It was the second game of his latest rehab stint.
"Depending on how bad the hamstring is, I don't want to write him off," Brad Ausmus told MLive.com. "But it'd be tough at this point."
The Tigers have not yet recalled Dirks from his rehab assignment, which is his third in less than two months. An announcement is expected Wednesday, a day after an MRI on the hamstring, the results of which has yet to be revealed. Dirks remains on the 60-day disabled list.
A stiff back that has plagued
The Thursday off-day will allow Ackley to rest his back for two straight days.
Davis has been on a mini-roll with seven hits in his last 21 at-bats.